The invention relates to transmission systems for video signals. More specifically the invention relates to low noise coupling devices to couple a shielded unbalanced video signal wiring to a balanced unshielded wiring.
Cable television (CATV) is transmitted over 75 ohm coaxial cable. Channels 2 through 60 are transmitted on the cable at frequencies ranging from 50 to 500 megahertz. Many homes and commercial buildings have previously installed telephone cable consisting of unshielded twisted pairs. Standard unshielded twisted pairs (UTP) of telephone cable have a characteristic impedance in the range of 100 ohms. Moreover, standard computer network wiring has a 100 ohm characteristic impedance which is used for telephone transmission as well. On occasion it is desirable to utilize cables of such types for transmission of video signals, this is especially true where standard cable television cable has not been installed or the cost differential to install UTP versus coaxial cable is significant.
There are difficulties associated with using the unshielded twisted pair (UTP) cable. First, the transmission through standard CATV coaxial cable is unbalanced. Any attempt to utilize the UTP cable with unbalanced signals will result in unacceptable radiation and attenuation of the CATV signals. Conventional techniques of matching and balancing the cables do not offer optimal performance across the broad frequency range utilized by CATV signals. Less than optimal matching can dramatically shorten the length of UTP cable that can be used to transmit such video signals without amplification.
Moreover, in that UTP is unshielded it is much more susceptible to noise or other RF interference. Conventional noise reduction techniques or devices are either not effective across the broad CATV frequency spectrum, are overly complex, and/or tend to cause excessive attenuation to the video signals.